Chapter 6-the rest Flashcards
Casting
- In the casting process, metals are first melted in a furnace.
- Alloying elements are then added and thoroughly mixed
(magnesium is added to aluminum to produce a stronger and
lighter alloy). - Oxide impurities and unwanted gasses are removed.
- The melt is then poured into a mold and chilled to solidify
recrystallization temp
Used in hot rolling where materials are heated to a high temp
% cold work
= [(initial metal thickness- final thickness)/ initial thickness] x 100
extrusion
metal workpiece (billet) is placed under high
pressure using a ram and forced through opening in a die
indirect extrusion
experiences lower friction on the billet and
therefore needs less power however has a limit on the applied
load
direct extrusion
produces products such as cylindrical rods, tubes,
and with some metals more irregular shapes
forging
metal is hammered into a desired shape
open die
Dies are either flat or
simple in geometry
closed die
Dies have upper
and lower impression – more complex
wire drawing
the starting rod or wire is drawn through
several drawing dies to reduce diameter
cold work of wire drawing
= (Change in cross-sectional area/ original area) x 100
elastic deformation
Atoms elongate resulting in overall elongation of the specimen. But
return to their original dimensions after tensile force is removed,
resulting in recoverable deformation
plastic deformation
Atoms break bonds and slip on each other.
Once slip occurs, atoms can not return to their original positions, resulting in permanent deformation.
normal stress σ
=(F -> avg uniaxial tensile force)/ (Ao -> original cross-sectional area) (lb/in^2 or N/m^2)
Normal strain ε
=Change in length/ original length
shear stress τ (parallel to surface)
=(S -> shear force)/ (A -> area of application)
shear stress γ (change in shape)
= amount of shear displacement a/ distance h over which stress acts = tan θ
linear elastic range
linear portion of the stress strain curve
yield point
The point on the curve when the elastic range ends
ultimate tensile strength σu
the largest stress the
specimen can take before fracture
Fracture point
The point on the curve where the specimen fails
Modulus of elasticity (E)
Stress and strain are linearly related in the elastic region. (Hooke’s law)
= stress/ strain
poisson’s ratio is always positive
= -[(w-wo)/ wo] / [(l-lo)/lo]
Yield strength
the stress at which
a material begins to experience
permanent or plastic deformation
% elongation fracture
a measure of ductility of a
material
= (final length- initial length)/ initial length
modulus of resilience Ur
measure of the amount of
energy needed to cause yield in the material
=1/2 (σy-Ey)
toughness
amount of energy required
to fracture the material. It is a measure of combination of strength and ductility
true stress and true strain
are determined based on instantaneous
cross-sectional area and length of the specimen
hardness
a measure of the resistance of a metal to localized
plastic deformation or indentation
slip bands
Plastic deformation in a single crystal results in formation of bands
slip
caused by application of shear stress on the grain.
The mechanism by which slip occurs is by formation of
dislocations and their movement
unit step of slip
created Once the dislocation reaches the
end of the grain
resolved shear stress (shimid’s law)
Tr=σ(cosλ cosθ)
critical resolved stress
the shear stress required to
cause slip in a single crystal
twin
part of atomic lattice is deformed and forms
mirror image of lattice next to it
hall-petch equation
=σy = σ0 + k/d^1/2
σy= yield strength
σ0 and k are constants
d is avg grain size diameter
solid solutions
addition of one or more impurity (solute) to
the host metal can increase the strength of metals
annealing
heat treatment
process applied to a strain-
hardened or cold-worked
metal to remove residual
stresses, grow new equiaxed
grains and soften the metal
three stages of the
annealing process are:
recovery, recrystallization and
grain growth
polgonization
Dislocations rearrange themselves to
form lower energy configurations
composite
combination of two or more individual materials designed to obtain more individual properties
composite phase types
– Matrix - is continuous
– Dispersed - is discontinuous and surrounded by matrix
Composite Types:
Particle reinforced
Fiber reinforced
structural
nano
Particle reinforced
– Types: large-particle and dispersion-strengthened
– Properties are isotropic
fiber reinforced
can be continuous (aligned) and discontinuous (aligned or random)
- Fibers very strong in tension
– Provide significant strength improvement to the
composite
– Properties can be isotropic or anisotropic
structural
- Laminates -
– stacked and bonded fiber-reinforced sheets - Sandwich panels
– honeycomb core between two facing sheets
pultrusion
- Continuous fibers pulled through resin tank to impregnate fibers with
thermosetting resin - Impregnated fibers pass through steel die that preforms to the desired shape
- Preformed stock passes through a curing die that is
– precision machined to impart final shape
– heated to initiate curing of the resin matrix
filament windind
– Continuous reinforcing fibers are accurately positioned in a predetermined
pattern to form a hollow (usually cylindrical) shape
– Fibers are fed through a resin bath to impregnate with thermosetting resin
– Impregnated fibers are continuously wound (typically automatically) onto a
mandrel
– After appropriate number of layers added, curing is carried out either in an
oven or at room temperature
– The mandrel is removed to give the final product
Ceramic Materials
Clay
glass
refractories
carbon
abrasives
cements
Die blanks
– Need wear resistant properties!
* Die surface:
– 4 μm polycrystalline diamond
particles that are sintered onto a
cemented tungsten carbide
substrate
refractories
- Materials to be used at high temperatures (e.g., in
high temperature furnaces)
Ceramics
the bonding between the atoms is generally covalent or ionic, and they are much stronger than the metallic bonds.
Hardness, thermal and electrical resistance are significantly higher than in metals.
Ceramics are available in single-crystal and polycrystalline form.
Finer grain size ceramics have higher strength and toughness.
phase change
the structure of a material
changes from one form to another.
phase
A region in a material that has a uniform structure, properties, and composition and maintains a distinct boundary with other unlike phases.
Ex: ice, water, vapor
equilibrium
The state of a system when all forces and energies are balanced resulting in a stable system with no tendency to change with time
phase diagram
Graphical representations of phases present in a material system at different temperatures, pressures and compositions.
Are developed based on assumption of equilibrium conditions resulting from slow cooling (equilibrium is approached but never fully maintained)
triple point
point at which the 3 stages of matter coexist
The number of phases that can
co-exist in equilibrium in a given system.
* P+F = C+2
P = number of phases that coexist in a system
C = Number of components
F = Degrees of freedom
cooling curve
Temperature of molten metal is recorded versus time as
it cools to room temperature,
Every time there is a phase change, there is a change in slope.
For a pure metal, the cooling curve shows a flat region (plateau) at a specific temperature where liquid transforms to
solid
In plateau region:
there is a mixture
of two phases in equilibrium.
In a phase diagram:
Created from a series of cooling curves.
* Above the “start” line, the alloy is all liquid.
* Below the “finish” line, the alloy is all solid.
In the region between start
and finish, a mixture of solid
and liquid exists
liquidius
The line that indicates start of
the solidification process
solidus
The line that indicates completion of the solidification process
binary isomorphous
system
a system where the two components are completely soluble in each other in both liquid and solid states (they do not from a third phase)
Tie line
In the mixture region (L + α), the composition of both liquid (L) and solid (α) phases at any temperature can be determined by drawing a tie line
Wo
Overall alloy composition
Wl
The composition at point L is the composition of liquid, in the
mixture.
Ws
The composition at point S is the composition of solid, in the
mixture.
The amount of each phase can be determined through application of the Lever Rule:
Wt fraction of solid phase = Xs= (Wo-Wl)/(Ws-Wl)
Wt fraction in liquid phase= Xl= (Ws-Wo)/(Ws-Wl)
